This invention relates to a short-range pulse-compression radar system particularly suitable for fabrication as an integrated circuit.
A need exists for a short-range radar system, which, for example, is suitable for automotive and other commercial applications. Such a system would be enabled to sense the proximity of other vehicles and objects, moving or stationary, within a range radius of about 0.15 meters to 30 meters or beyond. Radar systems having automotive application have been proposed in the prior art, including systems utilizing radar for automatic braking, as well as warning the operator of the vehicle of an impending danger or obstruction such as an animal or person in the roadway.
In addition to object range detection, it is also useful to be able to distinguish or resolve the distance between two objects in very close proximity (e.g. when they are approximately 15 centimeters apart). An intelligent radar detection system comprises a number (one or more) of sensors that operate as a transceiver for electromagnetic energy. The sensors typically transmit and receive electromagnetic energy of a defined frequency and power via an antenna over a defined spatial area. In turn, the system receives echo signals from partial reflections of any illuminated objects in that area. The ability of the sensor to resolve two or more objects closely situated in the illuminated area leads to the descriptive name of high-resolution radar (HRR).
The prior art also refers to high range resolution mono-pulse radar systems which are designated (HRRM). See, for example, a text entitled, xe2x80x9cRadar Handbookxe2x80x9d, Second Edition, by Merrill Skolnik, published by McGraw Hill, Inc. (1990). This text gives descriptions of radar systems, including high-resolution systems. As one can ascertain, there is a large potential market for automotive radars as well as for other commercial applications. Such applications include, but are not limited to, automatic door openers, sanitary facilities, electronic boundary detectors or fences, electronic camera focusing, navigation devices, parking aid sensors, and a host of other potential uses. However, in order to create a system for such markets, a technical solution must be provided that is not only capable of operating with the required degree of performance, but also offers a potential route to lower cost sensors. The price reduction should be able to take advantage of economies of scale and other established high volume manufacturing techniques. In addition, the sensor architecture should be sufficiently flexible to offer multiple operating modes to enable varying application and custom requirements depending upon the intended end use.
The prior art was cognizant of the need for low cost, high-resolution radars. Reference is made to U.S. Pat. No. 6,067,040 entitled, xe2x80x9cLow Cost High Resolution Radar for Commercial and Industrial Applicationsxe2x80x9d, issued on May 23, 2000 to K. V. Puglia. The patent describes a low cost, high-resolution radar based detection system, which has a pulse repetition frequency generator connected to first and second narrow pulse modulators. The system employs a transmit channel which is connected to the first narrow pulse modulator and emits pulse modulator carrier based transmit signals having a prescribed frequency and a prescribed duration. The receive channel is connected to the second narrow pulse modulator. There is a time delay circuit which delays the output of the second pulse modulator to the receive channel and a mixer which mixes a portion of one of the pulse modulated carrier based transmit signals reflected from an object with the output of the second narrow pulse modulator.
PCT application entitled, xe2x80x9cSensor for Measuring a Distance from an Objectxe2x80x9d No. WO 00/43801 having a priority date of Jan. 20, 1999 and filed for Martin Reiche describes a sensor for measuring the distance from an object. The apparatus includes an oscillator which generates a carrier signal. A first modulation switch modulates pulses on a carrier signal and generates a first pulse signal. The first pulse signal is emitted in the direction of the object. The first pulse signal is reflected by the object and delayed by a propagation time. A power divider positioned between the oscillator and the first modulation switch transmits the carrier signal to a second modulation switch. The second modulation switch modulates the pulses on to the carrier signal and generates a second pulse signal that is delayed by a variable delay. One compares the delay of the second pulse signal with the propagation time of the first pulse signal to detect the propagation time and determine the distance to the object.
An aspect of the invention is to increase the pass-band transmission loss of the modulation switches by providing for a third modulation switch which is positioned between the oscillator and the power divider. As one can see from the above-noted techniques, the typical operating scenario presented in the above systems are based on a combination of discrete circuit components combined with distributed transmission line elements on a soft substrate. These prior art approaches can lead to a combination of manufacturing tolerance issues and operating scenarios that compromise the performance of the sensor. It is understood that the design and assembly of the sensor based on discrete components leads to a relatively large device. The functional operation of the sensors is restricted for both size and cross constraints as each additional circuit block is relatively expensive to add. The use of distributed transmission line circuitry is a common technique for the design of high frequency microwave and millimeter circuits, but is based on the fundamental assumption that standing waves are present in the circuit. This assumption no longer holds true under short pulse conditions, and can lead to transient and short-term circuit effects that reduce the operating margin and compromise sensor performance. Lastly, the mid- to long-range operation of the short pulse sensor is not optimum due to two issues. The energy received by the sensor from partial reflection of the detected objects varies as an inversely proportional function of the fourth power of the object""s range. As the range increases, the ability of the sensor to detect objects rapidly decreases as a function of the greatly reduced energy incident upon, and reflected from objects. Conventionally, there are two limitations that restrict the amount of energy (power) that may be transmitted by the sensor: the ability to discriminate between two targets (range discrimination) is a function of the pulse-length in pulsed-radar systems, and the chirp or frequency modulation bandwidth in a CW radar system. A longer pulse length increases the amount of energy transmitted by the sensor with a consequent reduction in the ability of the sensor to discriminate between closely located objects. Also, the interval between pulses (or pulse repetition frequency (p.r.f)) may not be reduced indiscriminately to increase the transmitted energy for the need to maintain an unambiguous range measurement. In addition, the sensor is susceptible to in-band interference sources that produce and transmit electromagnetic energy in the same portion of the electromagnetic spectrum as that of the sensor. The forms of the interfering sources include CW or pulsed transmission by other systems, mutual interference from a second sensor or sensor system operating with the same or similar purpose, self-jamming through imperfect isolation between the transmit and receive port antennae, and wide-band thermal noise.
Thus, one can readily understand that these problems increase with such sensor devices that are used in, for example, the automotive industry. For example, hundreds of cars on a single highway may all be generating and receiving signals operative in the same radar range or at similar frequency bands.
In accordance with an aspect of the present invention; a variable length pulse is introduced that increases the transmitted energy of a sensor at longer ranges. The superimposition of a short duration phase coding on the expanded transmitted pulse is used to maintain the required range resolution of the sensor for the instances of longer pulse duration. The phase coding is also useful in increasing sensor immunity to interference from other sources. In addition, by making the specific phase-code variable as a function of range, the p.r.f. of the sensor can be increased without compromising range ambiguity. Other circuit functions that supplement and enhance this process include varying the pulse repetition frequency of the transmit sequencexe2x80x94both over time and as a function of the range-gate under observation, adding frequency modulation to the local oscillator within the sensor and a variable gain amplifier used to control and vary the amount of energy transmitted at any instance by the sensor. The receiver of the sensor includes a two step pre-detection integration process to ensure that the reflected energy captured by the sensor is as large as possible at a given instant to maximize the likelihood of a correct detection decision.
The increased functionality of the sensor according to the present invention can be addressed while simultaneously considering the issue of manufacturability and cost. A system embodying an aspect of the present invention incorporates circuit functions into either a single transceiver integrated circuit (IC) or a dual IC chipset comprising a separate transmitter and receiver IC, or a combination thereof. The high integration capability of integrated circuit processes allows several circuit functions to be located in close proximity on a single chip. In addition, the reduced circuit size and interconnect distance between components allows circuits to be designed using conventional analog and lumped circuit theory. This technique eliminates the need for distributed circuit design that is not ideal for short pulse transient conditions. The circuit is preferentially designed using balanced circuit configurations to maximize common-mode noise rejection, although single-ended circuit designs are also possible. The integrated circuit process that is preferred for accomplishing enhanced operation and increased circuit density is a Silicon Germanium (SiGe) process that includes both bipolar transistors and CMOS transistors as part of the same circuit (BiCMOS). Other suitable technologies that are also considered useful are SiGe bipolar only processes; and III-V processes such as GaAs or InP based MESFETs, pHEMTs, or HBT devices. It is understood that such integrated circuit techniques can be employed because of the system architecture and because of the way the system is implemented using a variable pulse length that basically increases the transmitted energy of the sensor at longer ranges.
An improved short-range radar system suitable for automotive and other short-range commercial applications for sensing the proximity of vehicles or objects within a radius ranging from about 0.1 meters to about 30 meters and beyond.
An aspect of the present invention is embodied in a system architecture for a short-range radar system which is capable of being implemented utilizing conventional integrated circuit techniques.
A further aspect of the present invention is the use of variable length phase codes whose code length may be varied as a function of range to provide increased immunity to interference sources; the ability to transmit longer pulses with a short-phase code to maintain short-range discrimination, and the ability to use the variable length code as a means of maintaining a relatively high p.r.f. compared to conventional radar sensors while still maintaining a high unambiguous range.